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Sunday, 20 November 2011

Resolution Bandwidth Filter #1

I acquired a couple of cheap 8-pole NDK 10P17D and 10P7D crystal filters which I intend to use as RBW filters. I don't know the actual specifications other than that they all have a centre frequency of 10.7 MHz. Their bandwidth is supposedly 17 kHz and 7 kHz.

I don't have any ready made PCBs for these filters at the moment as it turned out that their footprint is rather large and won't fit on a 1 x 1 SLIM. The picture above shows a top view of the home-brew enclosure I came up with. I used scrap FR4 for the walls and shielding and soldered it together to form a very crude box.This prototype is simply a perfboard which I coated with copper tape to get some kind of ground plane. I forgot to take pictures from the inside - but it's really not that exciting anyway.

A preliminary test revealed that the in/out impedance suggested by the seller was way off. These filters appear to be matched already, but for 100 ohms.

In this picture you can see how I used adhesive copper tape to seal the bottom of the filter.

After a long night of chasing data sheets I think I found something useful. Apparently the terminating impedance of this filter is 910 ohm/ 25 pF. On top of that these filters seem to have an integrated matching transformer.

I will design a proper PCB for this filter can type but for now this one has to do the trick.

This is a very hasty frequency response taken with a HP4395A to get a first idea of the filter properties.

Edit: I recorded the 2-port touchstone data for this filter with 1600 points. You can download it as a zip file from my Google docs: NDK10P17D_plain.zip